100 mV Polarization Criterion

Why the 100 mV Criterion Matters

The 100 mV criterion is important because corrosion control is not always best evaluated by a single fixed potential value. Soil conditions, coating condition, electrolyte chemistry, reference electrode limitations, shielding, current distribution, and structure behavior can make a fixed threshold difficult to interpret.

A structure may fail a fixed −850 mV_CSE polarized-potential criterion but still demonstrate enough cathodic polarization to satisfy a 100 mV criterion, if the governing requirement allows that interpretation and the data are technically valid.

This does not mean the 100 mV criterion is easier or weaker. In many field situations it requires more discipline because the evaluator must prove that the measured shift was caused by cathodic protection and represents the structure being evaluated.

Core Concept

Polarization

Polarization is the shift in structure potential caused by cathodic protection current. When CP current flows to a protected steel surface, the structure potential shifts in the cathodic direction.

Depolarization

Depolarization is the loss of that shift after CP current is removed or interrupted. As the structure loses polarization, the potential generally moves back toward a less protected or native condition.

Measured shift

The criterion is commonly evaluated by comparing a polarized condition to a native or depolarized condition. The difference between the readings must represent the effect of CP, not a change caused by the wrong current source, poor reference electrode placement, unstable readings, or unrelated electrical interference.

Different from −850 mV_CSE

The 100 mV criterion is not the same as the −850 mV_CSE criterion. The −850 mV_CSE approach evaluates an absolute potential threshold. The 100 mV approach evaluates a potential change caused by CP.

Common Ways the Criterion Is Evaluated

Polarization formation

Polarization formation compares a native or initial potential to a protected potential after CP has been applied. The key question is whether CP caused a sufficient cathodic shift.

Depolarization testing

Depolarization testing compares a polarized condition to a later depolarized condition after CP current is interrupted or disconnected. The key question is whether the structure loses at least 100 mV of polarization after CP is removed.

Coupon-based evaluation

Coupons may be used to represent a coating holiday or exposed metal surface. Coupon measurements can be useful, but only when the coupon is installed, connected, disconnected, and interpreted according to the applicable method.

Measurement Conditions That Matter

A calculated 100 mV shift is only meaningful if the compared readings are technically comparable. The same reference electrode type, representative test location, valid timing, and controlled current-source condition must be considered.

Remaining active current sources can invalidate the test. If other rectifiers, bonds, galvanic anodes, or foreign-current effects continue to polarize the structure, the measured depolarization may not represent the CP source being evaluated.

Timing also matters. Some structures depolarize quickly; others depolarize slowly. A premature or poorly documented reading can misrepresent the actual polarization change.

Calculation Example

A tank bottom has an instant-off potential of −760 mV_CSE. After CP current is removed and the structure depolarizes, the potential shifts to −640 mV_CSE.

The depolarization shift is 120 mV because the potential changed from −760 mV_CSE to −640 mV_CSE. If the test method is valid and the applicable requirement permits the criterion, the result may satisfy the 100 mV polarization criterion even though the instant-off value is not more negative than −850 mV_CSE.

How to Interpret Pass and Fail Results

A passing 100 mV result generally means the structure demonstrated the required polarization response under the tested conditions. It does not prove that every part of the structure is protected, and it does not eliminate the need to consider survey coverage, continuity, coating condition, shielding, and interference.

A failing result may indicate insufficient CP current, poor current distribution, depleted anodes, rectifier issues, continuity problems, shielding, an unstable reference electrode, or an invalid test setup. The next step should be technical troubleshooting, not blind adjustment.

Common Mistakes

1. Comparing readings from different test conditions.
   Why it is wrong: The calculated shift may not represent actual CP polarization.
2. Leaving other CP sources active during depolarization testing.
   Why it is wrong: Remaining current sources can continue polarizing the structure.
3. Assuming any 100 mV difference automatically passes.
   Why it is wrong: The shift must be caused by CP and measured under a valid method.
4. Using poor timing or undocumented timing.
   Why it is wrong: Depolarization is time-dependent and must be interpreted with context.
5. Treating the 100 mV criterion as interchangeable with −850 mV_CSE.
   Why it is wrong: One criterion evaluates potential level; the other evaluates potential change.

Study Checklist

• Can you define polarization and depolarization in CP terms?
• Can you calculate a 100 mV shift from two structure-to-electrolyte readings?
• Can you explain why current-source control is required?
• Can you explain how a structure can fail −850 mV_CSE but satisfy 100 mV polarization?
• Can you identify when a calculated shift is not technically valid?
• Can you document the measurement condition and timing clearly?

Practice Questions

1. What does the 100 mV polarization criterion evaluate?
2. What is the difference between polarization and depolarization?
3. Why must the compared readings represent the same structure and test condition?
4. Can a structure fail −850 mV_CSE but satisfy 100 mV polarization?
5. Why can remaining active current sources invalidate a depolarization test?

Related Pages

• Cathodic Protection Criteria
• −850 mV_CSE Criterion
• 100 mV Polarization Criterion
• Instant-Off Potential
• Structure-to-Electrolyte Potential
• Polarization Change Formula
• CP 3 Certification Study Guide
• CP Coupons